Respiratory syncytial virus (RSV) is the single most important viral pathogen causing acute lower respiratory- tract infections (bronchiolitis) in children. No vaccine or specific therapy is currently licensed to prevent and treat RSV infections. Exposure to environmental tobacco smoke (ETS) has been identified as a risk factor for the development of severe RSV infections, yet the mechanisms that determine ETS and infection co-morbidity are largely unknown. Recent studies support the notion that severity of clinical disease is driven by higher level of RSV replication in the airways and studies by our group have shown that expression and activity of superoxide dismutase (SOD) antioxidant enzymes are significantly decreased in the airways of infants with severe episodes of RSV bronchiolitis, particularly if infants had been exposed to ETS. Surprisingly, our new preliminary data have shown that SOD enzymes are able to inhibit RSV replication in cultured epithelial cells and in experimentally-infected mice. We propose the novel hypothesis that lung SOD enzymes have a previously unrecognized function as host antiviral gene products by limiting RSV replication and shortening viral shedding. Our secondary hypothesis is that expression and antiviral function of such enzymes are reduced/impaired in RSV-infected subjects who are exposed to ETS. In this exploratory project we propose to test these hypotheses by two Specific Aims. In Aim 1, the profile, relative abundance, and activity of SOD proteins will be analyzed in nasopharyngeal secretions (NPS) of infants with RSV infections of different clinical severity and with or without exposure to ETS. SOD levels and activity will be correlated with the concentration (i.e. viral titer/load) of RSV in NPS. In Aim 2, we will test the specific hypothesis that increasing of SOD activity in the lungs will blunt RSV replication and improve airway disease. For that, we will increase SOD expression in the lung by adeno-associated virus (AAV)-mediated gene transfer or by the use of salen-based EUK SOD mimetics, and measure RSV titer (as the main endpoint) in an experimental mouse model of RSV infection and co-exposure to ETS. The protective effect of SOD will be evaluated by examining clinical disease, lung inflammation/pathology, and viral-mediated airway hyperresponsiveness. This project may have important translational implications by suggesting new and more aggressive strategies to treat primary respiratory viral infections in high risk infants (i.e. those exposed to ETS). These therapeutic opportunities will b explored in a future R01 application based on the results of this exploratory project.